Can someone help me understand this processing workflow?

Pleiades Astrophoto PixInsight Processing SHO
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AmyWarble avatar
AmyWarble avatar
I have three nights of Eagle Nebula goodness captured in SHO+LRGB.  I wanted to try out the workflow depicted here, using SHO for the nebula and LRGB for the stars.

Now, I've done that combination before, but not using this workflow.  

If you look at the top row, just to the right of Plate Solve, it says "Spectro Narrowband Color Calibration".  What exactly is this step?  It's using the same icon as SPCC, but my understanding is that SPCC should not be used with narrowband.  So I'm not sure what I should be doing with this step.

Does anyone use this workflow?  What does this step mean?  And also, what is "Continuum Subtraction"?  Is this some kind of technology procured through the Stargate Program?

Or, is there a reason I should not use this workflow?  I'm not exactly set on using this, I'm just trying to learn new things.  Y'know?

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Rick Krejci avatar
Rick Krejci avatar
There's a Narrowband Filters Mode checkbox in SPCC, and you enter in your narrowband filter characteristics.

Continuum Subtraction attempts to remove the broadband component from the narrowband image, leaving only narrowband signal.   I use it to isolate Ha component of, say, a galaxy so when you add the Ha back in to a broadband image it will only enhance the narrowband signal and won't throw colors off in other areas of the galaxy.  It's in the Toolbox Script folder.

I do this flow mostly for emission areas, but I hadn't been doing the continuum subtraction on the right side.    May need to give that a try!
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Yannis Doukakis avatar
Yannis Doukakis avatar
Few comments on the above workflow.
a) I do not like doing BlurX separately on channels as in some (rare) cases I got color fringing on stars. First I merge the image and then BlurX. If I need the NB channels separately, I merge in RGB (HSO/SHO/… does not matter), do BlurX and then separate the channels back into H,S,O
b) Never got decent results doing continuum subtraction on starless images - maybe my mistake
c) I avoid using BlurX before SCC, as BlurX does not modify the low-level stars (close to the noise). It makes the high-level stars sharper (which means higher value). So I believe that it skews the linearity of the image disturbing the SCC process. I prefer first to do SCC and then BlurX
d) Finally you do an SCC then you separate the channels, then you adjust color calibration. If there is no vast difference in the filter widths or exposure times, what does the SCC offer?
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Dominik Weinbrenner avatar
Dominik Weinbrenner avatar
This workflow has questionable practices.

When imaging emission nebulae and the target is an SHO image, why should you bother with subtracting continuum. Does not make any sense to me.

Using BXT on the individual masters is also not needed, but can work if one master has substantially worse FWHM. 

I also find normal color calibration with clever ROIs far better for SHO than SPCC when it comes to balancing out the channels.
Rick Krejci avatar
Rick Krejci avatar
Dominik Weinbrenner:
...

I also find normal color calibration with clever ROIs far better for SHO than SPCC when it comes to balancing out the channels.

If I don't have RGB stars, I'll often use SPCC NB Optimize for stars to yield better balanced stars from the narrowband image before I remove them
Noah Tingey avatar
Noah Tingey avatar
Not sure why the guide you're following recommends doing continuum subtraction for an image that isn't doing narrowband combination with broadband data. Maybe there's some logic to it that I'm not seeing, but it seems like a weird recommendation. In addition to the other dubious recommendations that other people in this thread have mentioned. 

Not to self-plug super hard but I did publish a little article on my website that goes through a SHO + RGB stars workflow that I feel is more sensible. https://nrstellar.com/blogs/articles/narrowband-editing-workflow-for-pixinsight
AmyWarble avatar
AmyWarble avatar
Thanks fellas, I'll stick to my regular workflow for now smile
AmyWarble avatar
AmyWarble avatar
For the curious, I found this workflow here:   https://astro-photographie.fr/traitement_pixinsight.html

I consider these as a kind of "map" as I explore and learn PixInsight's tools.

What is the community consensus opinion on those workflows?  I see them referenced sometimes.
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Dominik Weinbrenner avatar
Dominik Weinbrenner avatar
Rick Krejci:
Dominik Weinbrenner:
...

I also find normal color calibration with clever ROIs far better for SHO than SPCC when it comes to balancing out the channels.

If I don't have RGB stars, I'll often use SPCC NB Optimize for stars to yield better balanced stars from the narrowband image before I remove them

Valid thing to do, but that is an entirely different case. OP‘s guide uses SPCC for the nebula structures, which is inferior to CC in terms of balancing.
max storn avatar
max storn avatar
    [*]
    Stack S, H, O → Combine SHO → Color Cal → Remove stars → Subtract continuum → Enhance.
    [*]
    Stack R, G, B → Combine RGB → Color Cal → Extract RGB stars.
    [*]
    Stretch and screen RGB stars over final enhanced SHO image.
Rick Krejci avatar
Rick Krejci avatar
Dominik Weinbrenner:
Rick Krejci:
Dominik Weinbrenner:
...

I also find normal color calibration with clever ROIs far better for SHO than SPCC when it comes to balancing out the channels.

If I don't have RGB stars, I'll often use SPCC NB Optimize for stars to yield better balanced stars from the narrowband image before I remove them

Valid thing to do, but that is an entirely different case. OP‘s guide uses SPCC for the nebula structures, which is inferior to CC in terms of balancing.

Agreed
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